Generally, electrically driven injection molding machines have a built-in load cell located inside a screw drive mechanism to detect a pressure to be applied to a screw.
FIGS. 3 and 4 show a known injection molding machine having such a built-in load cell.
As shown in FIG. 3 an injection molding machine 60 includes a guide coupling 53 which is slidably supported by a plurality of guide shafts 51 and to which linear movements from a ball screw mechanism 52 is transmitted. The machine 60 also includes a screw coupling 54 which is coupled to a rear end of a screw 55 and to which rotary movements are transmitted. The screw coupling 54 is rotatably supported by the guide coupling 53. In addition, a washer type load cell (pressure detector), which comprises an inner tubular portion 62, an outer tubular portion 63, and an intermediate distorting portion 64, is interposed between the screw coupling 54 and the guide coupling 53. The load cell 61 and the guide coupling 53 are secured to the screw coupling 54 by a bearing nut 56, together with a thrust bearing 58 and an angular ball bearing 59. Both of the bearings 58 and 59 interpose the load cell and the guide coupling 53 therebetween.
As a result, once a pressure from the screw 55 is provided to the inner tubular portion 62 through the screw coupling 54 and the thrust bearing 58, the intermediate distorting portion 64 generates a distortion which corresponds to the applied pressure because the outer tubular portion 63 is regulated by the guide coupling 53. This pressure is detected by distortion gages mounted on the intermediate distorting portion 64.
However, in the injection molding machine 60, a torque of the bearing nut 56 acts on the outer tubular portion 63 through the intermediate distorting portion 64. As a result of this torque, an initial distortion is generated at the load cell 61, thereby causing the bearing height of the thrust bearing 58 and the thickness of the load cell 61 to be decreased as the injection force increases. As a result, a pre-load to the angular ball bearing 59 is decreased in proportion to such decrease, thereby impairing linearity in the detection characteristic (output characteristic) of the load cell 61 in accordance with the magnitude of the injection force. This leads to incorrect pressure detections, making their corrections difficult. In addition, deviations from the zero point are caused not only by reversible variations in the pre-load due to secular changes derived from factors such as temperature variations and different thermal expansion coefficients among materials, but also by irreversible variations in the pre-load caused by the slackening of the bearing nut 56 due to repeated use. Such deviations bring about a reduction in detection accuracy, reliability, and the like.
FIG. 4 shows an injection molding machine 70 in which the above described shortcomings are overcome. The injection molding machine 70 shown in FIG. 4 is disclosed in Japanese Patent Unexamined Publication No. 27921/1989 and is constructed in such a manner that the load cell 61 is interposed between the guide coupling 53 and a ball nut 57 of the ball screw mechanism 52. With this construction, the influence from the pre-load applied from the bearing nut 56 is eliminated.
Although the injection molding machine 70 is free from the influence from the pre-load, it is affected by the frictional resistance of the guide coupling 53 and the like. That is, although the guide coupling 53 is slidably supported by the guide shafts 51, there exists not only certain friction but also error factors attributal to the guide shafts 51, i.e., variable error factors such as their thermal expansion and fixed error factors such as their machining accuracy and their parallelism during assembling. Consequently, the magnitude of friction varies in accordance with the position of the guide coupling 53 and the time which has elapsed from the start of operation. As a result, the injection molding machine 70, like other known machines, is not free from impairment in detection accuracy. Further, the installation of the load cell 61 outside of the guide coupling 53 requires a mounting screw hole, thereby leading to the disadvantage that the load cell 61 is large in structure. The elements and components in FIG. 4 which are the same as in FIG. 3 are designated by the same reference numerals so that its construction can be clearly viewed relative to FIG. 4.